Power supply

Abstract

A power supply for an incandescent lamp by which sufficient power is supplied to the lamp to enable the lamp to reach its desired color temperature without exceeding the break over voltage of its filament even though the peaks of the alternating current voltage applied to the power supply exceed the break-over voltage of the filament of the lamp. This is achieved in a preferred embodiment by full-wave rectification of the alternating current voltage applied to the power supply to produced an unfiltered voltage comprising a series of half-cycle sine waves of a particular polarity. This voltage is applied to the lamp. However, when the voltage of each half cycle is above the breakover voltage of the lamp, the lamp is switched out of the circuit, so that only those portions at the beginning and end of each half cycle which are below a particular voltage are actually applied to the lamp.

We claim: 
     
       1. A power supply for an incandescent lamp, or the like, comprising: an input circuit for supplying a varying voltage to said lamp in a series of cycles; a controllable switch connected in series with said lamp; and control circuit means connected to said switch for causing said switch to conduct only during intervals at the beginning and end of each of said cycles of said varying voltage and when said varying voltage is below a predetermined threshold, in which said input circuit is connected to an alternating current line and includes a full-wave rectifier for supplying said varying voltage to said lamp in a series of sine wave half cycles of a selected polarity. 
     
     
       2. A power supply for an incandescent lamp, or the like, comprising: an input circuit for supplying a varying voltage to said lamp in a series of cycles; a controllable switch connected in series with said lamp; and control circuit means connected to said switch for causing said switch to conduct only during intervals at the beginning and end of each of said cycles of said varying voltage and when said varying voltage is below a predetermined threshold, in which said control circuit means includes a comparator for producing an output having a first state when the varying voltage from said input circuit is below a predetermined reference level and having a second state when the varying voltage from said input circuit is above said predetermined reference level, so as to cause said controllable switch to switch to its conductive state only when the output from said comparator is in a selected one of said first and second states so as to cause said lamp to be energized or fly during predetermined portions of the varying voltage at the beginning and end of each cycle thereof. 
     
     
       3. A power supply for an incandescent lamp, or the like, comprising: an input circuit for supplying a varying voltage to said lamp in a series of cycles; a controllable switch connected in series with said lamp; and control circuit means connected to said switch for causing said switch to conduct only during intervals at the beginning and end of each of said cycles of said varying voltage and when said varying voltage is below a predetermined threshold, in which said controllable switch comprises at least one field-effect transistor. 
     
     
       4. A power supply for an incandescent lamp, or the like, comprising; an input circuit for supplying a varying voltage to said lamp in a series of cycles; a controllable switch connected in series with said lamp; and control circuit means connected to said switch for causing said switch to conduct only during intervals at the beginning and end of each of said cycles of said varying voltage and when said varying voltage is below a predetermined threshold, in which said controllable switch comprises at least one insulated gate bipolar transistor. 
     
     
       5. A power supply for an incandescent lamp, or the like, comprising: an input circuit for supplying a varying voltage to said lamp in a series of cycles; a controllable switch connected in series with said lamp; and control circuit means connected to said switch for causing said switch to conduct only during intervals at the beginning and end of each of said cycles of said varying voltage and when said varying voltage is below a predetermined threshold, in which said control circuit means includes a comparator for producing an output having a first state when the varying voltage from said input circuit is below a predetermined reference level and having a second state when the varying voltage from said input circuit is above said predetermined reference level, so as to cause said controllable switch to switch to its conductive state only when the output from said comparator is in a selected one of said first and second states so as to cause said lamp to be energized only during predetermined portions of the varying voltage at the beginning and end of each cycle thereof, and in which said comparator has first and second input terminals and an output terminal, and which includes first circuitry connecting said first input terminal to said input circuit and second circuitry connecting said second input terminal to a source of reference potential. 
     
     
       6. The power supply defined in claim 5, and which includes line voltage compensating circuitry included in said second circuitry for compensating for fluctuations in the voltage of the sine wave half cycles from said input circuit. 
     
     
       7. The power supply defined in claim 5, and which includes symmetry compensating circuit connected from the output terminal of said comparator to the second input terminal for compensating for lack of symmetry in the voltage waveform delivered to said lamp. 
     
     
       8. The power supply defined in claim 7 in which said symmetry compensating circuitry includes a potentiometer for adjusting the symmetry of the voltage waveform delivered so said lamp. 
     
     
       9. The power supply defined in claim 2, and which includes an intermediate voltage turn-on delay circuit connected to the output terminal of said comparator for establishing the voltage applied to said switch by said comparator at a predetermined intermediate value whenever the output of said comparator exceeds a predetermined threshold. 
     
     
       10. The power supply defined in claim 2, and which includes an over-current shutdown timer circuit connected to said switch for responding to current flow through said switch in excess of a predetermined threshold to remove the voltage applied to said switch from said comparator for a predetermined time interval.